1. Field of the Invention
The present invention relates to a producing method for zirconia hollow particles in which a zirconia powder mutually combines to form a porous shell layer thereby constructing a hollow structure, and more particularly to a method for obtaining zirconia hollow particles having a high mutual combining strength of zirconia powder and showing an absolutely spherical shape.
2. Related Art
A composite material formed by dispersing a ceramic powder in a matrix such as a metal is widely employed for example for the purpose of a weight reduction and a strength reinforcing. Also for a further weight reduction, there are recently utilized ceramic hollow particles in which the ceramic powder mutually combines to form an approximately spherical porous shell layer with a hollow interior. Also in light-weight components for an automobile, a weight reduction is intended by dispersing a zirconia powder, or a zirconia hollow powder in a metal.
Ceramic hollow particles have commonly been produced by covering an entire surface of a resin powder of a large size constituting a core material with a powder layer of a ceramic powder of a grain size smaller than that of the resin powder thereby forming a precursor, and eliminating the resin powder from the precursor to cause the ceramic powder to mutually combine thereby forming a porous shell layer. For example, there is known a method of producing alumina hollow particles by contacting a highly water-absorbing polymer powder in a swelled state by water absorption and an alumina powder to form a powder layer of the alumina powder on an entire surface of the highly water-absorbing polymer powder thereby forming a precursor, and drying at a high temperature or calcining the precursor thereby eliminating the highly water-absorbing polymer and constructing a hollow structure (cf. Unexamined Japanese Patent Publication Hei. 10-258223).
However, in the producing method described in the publication described above, in the precursor schematically shown in FIG. 5, an alumina powder 12 is merely attached to the surface of a resin powder 10 and is easily peeled off from the resin powder 10 at the high-temperature drying or at the calcining so that the powder layer is difficult to retain uniformly. Besides, as the resin powder 10 causes a thermal expansion or a gasification by the high-temperature drying or by the calcining, the alumina powder 12 receives an outward pressure whereby the powder layer tends to be disintegrated.
A similar situation occurs when the aforementioned producing method is applied to the preparation of zirconia hollow particles, and a uniform porous shell layer is difficult to obtain by the peeling of the zirconia powder or by the disintegration. For the purpose of further weight reduction, there are also desired fine zirconia hollow particles for example of a grain size of 20 μm or less, and a zirconia powder of submicron range has to be used for that purpose. It is further difficult to maintain a uniform powder layer and to form a satisfactory porous shell layer with such fine zirconia powder.